With progressive miniaturization in micro- and nanotechnology, and especially in semiconductor technology, transmission electron microscopy (TEM) of internal interfaces on ion-thinned cross sections is becoming more and more important. Targeted preparation of a selected object position or site, i.e. a specimen or sample lying on or in a sample position or locus of the substrate, is necessary for this. In future MOS components, for example, samples with the gate lengths of from 10 nm to 100 nm and gate oxide thicknesses of a few atomic layers will need to be prepared in a targeted way for high-resolution TEM.
In many areas of materials science, target preparation is also increasingly required on particular micro-regions, for example phase transitions, structure defects, heat influx zones, crack edges, interfaces etc. There are likewise not yet any suitable methods for the targeted preparation of samples for subsequent TEM in the area of the interface between materials and biological tissue, as occur for example in implants.
In order to prepare samples for such electron microscopic examinations, in particular for examinations with a transmission electron microscope (TEM), it is necessary to separate them from their substrate and mechanically fix them.
To this end, it is known from the prior art to separate the sample from the substrate with the aid of a high-energy focused ion beam (FIB) by physical etching or chemically assisted etching. The sample is subsequently taken from the substrate with the aid of a probe, either under a light microscope or still in the FIB device, and then fastened with the aid of this probe on a sample support; see for example U.S. Pat. No. 6,538,254 B1.
US 2003/0089860 A1 describes for example a method in which the sample is separated by FIB from a semiconductor substrate, the position where the sample is to be taken being imaged with the aid of a scanning electron microscope (SEM). The separated sample is removed with the aid of an electrostatically acting manipulator.
US 2002/0121614 A1 describes a method in which the sample is first fully separated from the substrate in the FIB device, before it is fixed on the probe. The probe may in this case be joined to the sample by ion beam induced deposition (IBID) or electron beam induced deposition (EBID) of material from a so-called precursor, by adhesives or electrostatically. The sample is then, for example, transferred onto a sample support and fastened there by material deposition.
WO 02/095378 also describes a method in which the sample is both imaged and separated from the substrate with the aid of FIB. Before separation, a probe is fastened on the sample position by IBID. With the aid of a micromanipulator, the probe with the separated sample is then removed from the substrate and the sample is placed on a TEM support grid and fixed there by IBID. The probe is subsequently separated from the sample with the aid of the ion beam.
DE 42 26 694 discloses a comparable method, in which the sample position is imaged via secondary electron emission in a scanning ion microscope (SIM). The separated sample is fastened on the probe by IBID and can then be observed under an SEM and further processed with an attenuated ion beam, in order to thin the sample for the TEM. The probe comprises a small thin probe head section on which the sample is fastened, and a thick holding section where it is fastened on a micromanipulator. The tip has a diameter of less than 10 μm and the holding section designed as a plate has a thickness of more than 50 μm.
Overwijk et al., “Novel scheme for the preparation of transmission electron microscopy specimens with a focused ion beam”, J. Vac. Sci. Technol. B 11(6), 1993, pages 2021-2024 disclose a method in which the sample is likewise imaged and prepared in an FIB device. In order to protect the surface of the sample when the sample is being separated by FIB, the sample is provided by IBID with a tungsten layer which also serves as a mask during the separation. After separation of the sample from the substrate, the substrate with the sample, which is contained in a recess produced by FIB in the substrate, is taken from the vacuum chamber and placed under a light microscope. There, the separated sample is then held via adhesion by a needle and placed on a TEM support by careful manipulation.
All the methods described so far, however, have the disadvantage that the sample is damaged (amorphization) or contaminated (ion implantation) by the high ion energy which is conventionally more than 30 keV. It is then only limitedly suitable for an examination by means of TEM.
Moreover, the damage takes place not only when separating the sample from the substrate by means of FIB but already when looking for the sample position, i.e. when imaging the substrate section containing the sample position in the scanning ion microscope. Although the sample damage is reduced with lower ion energies, the resolution of the imaging is nevertheless greatly reduced with a low ion energy, so that targeted finding of an intended sample position is no longer possible.
Mechanical handling of the prepared sample represents a further problem. This is because after preparation, the minute extremely fragile samples have to be manipulated onto a TEM support grid, which often leads to loss of the elaborately prepared and expensive samples. Often, there is also a poor thermal and electrical contact between the sample and the support grid, which leads to imaging artifacts by charging and heating of the sample.
It is also generally known from the article by Matsui and Mori: “New selective deposition technology by electron beam induced surface reaction” in J. Vac. Sci. Technol B4(1), 1986, pages 299-304, to deposit a tungsten layer on a substrate within a vacuum chamber with the aid of an electron beam (EBID), to which end a precursor, here for example WF6, is introduced into the vacuum chamber via a gas nozzle.
The invention therefore relates to a method for preparing a sample for electron microscopic examinations, in particular with a transmission electron microscope (TEM), having the steps:                a) a substrate containing the sample to be prepared on a sample locus is provided in a vacuum chamber,        b) a protective layer is applied onto a surface of the sample locus,        c) the sample located under the protective layer is separated from the substrate by an ion beam, the protective layer acting as a mask, and        d) within the vacuum chamber, the separated sample is removed from the substrate.        
Such a method is known from U.S. Pat. No. 6,188,068 D1. In the known method, the sample is cut free by FIB and then joined either to a TEM sample support and removed from the substrate using it, or taken using a probe on which the sample is held in a suitable way. The sample is then placed on a TEM sample holder.
The inventors of the present application have recognized that precisely the combination of a protective layer and handling still in the vacuum chamber avoids many problems which are prevalent in the prior art. The protective layer significantly reduces the risk of damage to the sample during separation from the substrate, while handling still in the vacuum chamber avoids mechanical damage. In the vacuum chamber, it is now possible to take the separated sample with great reliability and reproducibility, for example by adhesion. The loss of sample during handling, often to be observed in the prior art, is thus avoided.
Although Overwijk et al. also describe manipulation of the sample by using adhesion on a needle tip, this is nevertheless carried out under a light microscope, i.e. not in a vacuum. The inventors of the present application have now recognized that adhesion can be controlled better in a vacuum and is more reproducible than under atmospheric conditions. According to the inventors' understanding, this is attributable to the varying humidity under atmospheric conditions. In fact, the sample will adhere better or worse on the needle depending on the degree of humidity. In a vacuum, the moisture film required for the adhesion can now be provided in a controlled way without dependence on external influences.
Another advantage with this method is that the protective layer can be used as a mask so that, instead of focused i.e. high-energy ion beams, it is possible to use low-energy ion beams in the manner of an ion shower for the separation, which have a smaller position resolution than the high-energy ions in FIB. The position resolution required for separating the sample is achieved here by the protective layer mask, which has an even higher position resolution than FIB. This electron beam deposition of the protective layer also already avoids many of the aforementioned disadvantages. This is because when the protective layer is being deposited, the surface of the sample is not damaged by the electrons which can be used according to the invention, as is caused by the high-energy ion beams used for this in the prior art.